Current fuse and method of making the current fuse

ABSTRACT

In order to provide a current fuse with high solderability without containing harmful materials, solder chips containing 30 to 60 percent by weight of zinc, 0.1 to 2 percent by weight of copper, 0.1 to 1 percent by weight of nickel, and the remainder percent by weight being tin, or further containing 0.01 to 0.5 percent by weight of aluminum are inserted into the interior of the electrodes before pressing the electrodes into the ends of the substrate of the fuse, the exterior of the electrodes is heated to melt the solder chips, thereby connecting between the electrodes and the fuse wire.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/883,032 filed on Jul. 2, 2004, currently pending. Thedisclosure of U.S. patent application Ser. No. 10/883,032 is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free current fuse, which issuitable for electronic equipment and electronic components, forexample, and a method of making the current fuse.

2. Description of the Related Art

Various kinds of compact fuses, which are surface-mountable on a printedcircuit board or the like, have been proposed. For example, a fuse inwhich a fuse element stretches in a hollow space within a rectangularcase formed by attaching a ceramic casing main body and a lid has beendisclosed in Japanese Patent Application Laid-Open No. 8-222117.

In the case of this compact fuse, a fuse element and tin-lead solder,which has been adhered to the interior of metallic cap in advance, aresoldered.

On the other hand, tin-lead solder is essential for fabricating orassembling electronic equipment; however, it contains lead which isharmful to the human body and the like. Therefore, solder not containingharmful lead is desired by electronic equipment industries or relatedindustries, and various kinds of lead-free solder have been proposed.For example, a lead-free solder alloy containing 7 to 10 percent byweight of zinc (Zn), 0.01 to 1 percent by weight of nickel (Ni), and thebalance consisting of tin (Sn) has been described in Japanese PatentApplication Laid-Open No. 9-94688.

On the other hand, a solder material that has been disclosed in JapanesePatent Application Laid-Open No. 2000-15478 is developed consideringenvironmental issues after discarding electronic equipment, and is alead-free solder material containing tin, 3 to 18 percent by weight ofzinc in proportion to that tin and a small amount of additives (e.g.,nickel).

However, when solder containing 10 percent by weight of tin and 90percent by weight of lead is used to manufacture the fuse described inJapanese Patent Application Laid-Open No. 8-222117, a ceramic substrateabsorbs heat generated when that solder melts. Therefore, a temperatureto heat metallic caps must be set at approximately 400 degrees C., whichis approximately 100 degrees C. higher than the melting point of solder.

Such temperature degrades plating on the surface of metallic caps,resulting in deterioration of solderability when mounting a fuse on asubstrate.

On the other hand, the above-described lead-free tin-zinc-nickel solderis used for mounting a fuse on a substrate. That solder used in theinterior of component causes a problem of deterioration of quality, suchas break of a fuse element when the solder within each component meltsin a reflow process when mounting the fuse on the substrate.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the above-describedproblems, and aims to provide a current fuse, which has solderabilitysuitable for electronic equipment and electronic components, and doesnot have an adverse influence on the environment including the humanbody, and a method of manufacturing the current fuse.

The present invention has a configuration to achieve the above-describedobjectives. In other words, a current fuse includes a fuse wire andelectrodes which are soldered using lead-free solder, wherein thelead-free solder contains at least 30 to 60 percent by weight of zinc,0.1 to 2 percent by weight of copper, and the remainder percent byweight of tin.

Furthermore, a current fuse includes a fuse wire and electrodes whichare soldered using lead-free solder, wherein the lead-free soldercontains at least 30 to 60 percent by weight of zinc, 0.1 to 1 percentby weight of nickel, and the remainder percent by weight of tin.

Furthermore, a current fuse includes a fuse wire and electrodes whichare soldered using lead-free solder, wherein the lead-free soldercontains at least 30 to 60 percent by weight of zinc, 0.1 to 2 percentby weight of copper, 0.1 to 1 percent by weight of nickel, and theremainder percent by weight of tin.

Furthermore, a current fuse includes a fuse wire and electrodes whichare soldered using lead-free solder, wherein the lead-free solder havingany one of the above-described composition further contains 0.01 to 0.5percent by weight of aluminum.

Furthermore, a current fuse, including: a fuse wire; a container for thefuse wire; and cap electrodes that are attached to both ends of thecontainer; wherein the fuse wire is soldered to the electrodes usinglead-free solder that is provided in the interior of the electrodes, andthe lead-free solder has any one of the above-described compositions.

Even further, a current fuse includes a fuse wire and electrodes whichare soldered using lead-free solder, wherein the lead-free soldercontains at least 30 to 60 percent by weight of zinc and the remainderpercent by weight of copper and tin.

In addition, when manufacturing a current fuse, a fuse wire andelectrodes are soldered using lead-free solder at a temperature betweena solid phase temperature and a liquid phase temperature of thelead-free solder. Alternatively, a fuse wire and electrodes are solderedin an atmosphere of oxygen using lead-free solder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a chip-type fuse according to anembodiment of the present invention;

FIG. 2 is an external view of a current fuse, which has a substrateformed by combining a casing main body and a lid according to theembodiment;

FIG. 3 is an external view of a current fuse when covered by electrodesaccording to the embodiment;

FIG. 4 is a graph showing a comparison of changes in resistance value ofa fuse using tin-zinc type solder when changing the zinc content ratioaccording to the embodiment;

FIG. 5 is a graph showing a comparison of changes in resistance value ofa fuse using conventional solder and a fuse using solder according tothe embodiment;

FIG. 6 is a table showing the evaluation results from changing thecontent ratios of copper, nickel, and aluminum included in tin-zincsolder according to the embodiment; and

FIG. 7 is a flowchart describing the steps of making the current fuseaccording to the embodiment.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is described in detail forthwithwhile referencing the attached drawings. FIG. 1 is an exploded blockdiagram of a chip-type fuse according to the embodiment of the presentinvention. The chip-type fuse shown in the drawing is configured bystretching a fuse in a hollow space of a ceramic substrate. Thissubstrate is formed by attaching a ceramic lid 3 to a ceramic casingmain body 2.

At the center of the casing main body 2, an opening 9 which is openingupward is provided. At both ends of the casing main body 2, concaveportions 11 communicating with that opening 9 are provided.

On the respective bottoms of the concave portions 11, a single-threadgroove 8 is formed. A soluble fuse wire (fuse element) 1 is aligned withthis groove 8, and stretching across the opening 9.

Note that when stretching the fuse wire 1 between the concave portions11, a constant tension is applied to that fuse wire 1. In this case, thefuse wire 1 may be temporarily attached to the groove 8 using a resin orthe like in order to maintain that tension.

In this way, the lid 3 is attached to the opening 9 in the casing mainbody 2 in which the fuse wire 1 stretches. Both ends 13 of that lid 3are shaped so as to fit in the concave portions 11 (the widths and thedepths of the ends 13 are slightly smaller than those of the concaveportions 11). In addition, the center of the lid 3 almost completelycovers the opening 9 (the length and the width of the center of the lid3 are slightly smaller than those of the opening 9).

In this way, the exterior of the substrate formed by combining thecasing main body 2 and the lid 3 is almost rectangular. The exterior ofthe fuse in this case is shown in FIG. 2. As shown in FIG. 2, when thecasing main body 2 and the lid 3 are combined, a bent portion 10 of thefuse wire 1 protrudes from each end of the substrate, respectively. Thatbent portion 10 is connected to a metallic cap as described later.

Each of metallic caps (electrodes) 4 is made of copper or a copperalloy, for example, and has a shape where one side (which faces thesubstrate) of a cube is opened so as to cap the end of the substrate asshown in FIG. 2. In addition, solder chips 5, each having a compositionto be described later, are inserted into the respective interiors 6 ofthe electrodes 4 before capping the ends of the substrate with theelectrodes 4 (see FIG. 1). The electrodes 4 into which the solder chips5 are inserted, respectively, then cap both ends of the substrate (bypressing in and attaching).

Note that not only solder chips may be provided to the interior 6 ofeach electrode 4, but molten solder or creamy solder may alternativelybe provided to the caps.

FIG. 3 shows the exterior of the fuse over which the electrodes 4 arefit as described above. As shown in FIG. 1, a portion (heating position)7 indicated by a dashed line in the electrode 4 of the above-describedfuse is heated using a heater, for example, at a temperature of 350degrees C. for approximately three seconds to melt the solder chip 5,which has been inserted into the interior of the electrode 4 in advance.

As a result, the solder chips 5 are soldered (fusion bonded) to theabove-described bent portions 10 of the fuse wire 1, resulting in anelectrical connection between the fuse wire 1 and the electrodes 4.

As described above, the casing main body 2 and the lid 3 of thechip-type fuse according to the embodiment are formed so that the outerdimensions of the lid 3 are slightly smaller than the internaldimensions of the casing main body 2. This facilitates alignmenttherewith. More specifically, they are attached by inserting the lid 3perpendicularly to the casing main body 2 during assembling.

As a result, when the casing main body 2 and the lid 3 are attached, theends 13 of the lid 3 never protrude from the concave portions 11 of thecasing main body 2. This allows smooth pressing in and attaching of theelectrodes 4 to the ends of the substrate, and establishment ofelectrical connections between the fuse wire 1 and the electrodes 4.

Note that the lid 3 is attached to the casing main body 2 by bondingthem together with an adhesive such as an epoxy and heating at a giventemperature for a given period of time (e.g., at a temperature of 150degrees C. for approximately 15 minutes). The electrodes 4 are pressedin and attached after the adhesive has hardened.

In addition, the electrodes 4 of the chip-type fuse according to theembodiment are also soldered to land patterns on a printed circuit boardwhen mounting the fuse on the board. Subsequently, to facilitate andsecure soldering to the printed circuit board when the fuse is mountedon that board, although not shown in the drawing, the electrodes 4 areformed, for example, so that one of the four sides of the pressed in andattached electrodes 4 protrudes from the sides of the substrate of thechip-type fuse for just the thicknesses of the electrodes 4. The otherthree sides of the electrodes 4 are aligned with, that is, in plane withthe sides of the substrate of the chip-type fuse.

In this way, aligning the sides of the electrodes 4 with those of thesubstrate and flattening them facilitates usage of a vacuum chuck whenmounting the chip-type fuse on the printed circuit board, and alsofacilitates setting a seal on the fuse main body.

Next, a solder chip used for the chip-type fuse according to theembodiment is described in detail.

In the case of the fuse according to the embodiment, high temperaturetin-zinc type solder is used as a solder chip to connect the fuse wire.Heat resistance (rate of change in the resistance value) of solder,which contains 30 to 60 percent by weight of zinc (Zn) considered to bean appropriate percentage of zinc content, is measured. The exemplarymeasurement results are shown in FIG. 4.

Note that a characteristic of the conventional tin-lead solder(containing 90 percent by weight of lead) is shown on the far-rightportion of FIG. 4 for comparison. In FIG. 4, numerals preceding Zn arepercentages of zinc (Zn) content.

The higher the ratio of zinc (Zn) than tin (Sn), the higher the meltingpoint, resulting in improvement in solder heat resistance. However, toomuch zinc (Zn) leads to difficulty in working. On the other hand,shortage of zinc (Zn) leads to high fluidity and deterioration of solderheat resistance.

As shown in FIG. 4, even considering the above-mentioned conditions,high temperature tin-zinc type solder containing 30 to 60 percent byweight of zinc is available.

More specifically, lead-free solder containing 30 to 60 percent byweight of zinc, 0.1 to 2 percent by weight of copper, 0.1 to 1 percentby weight of nickel, and the balance consisting of tin is used.

Alternatively, lead-free solder containing 30 to 60 percent by weight ofzinc, 0.1 to 2 percent by weight of copper, and the remainder being tin,or lead-free solder containing 30 to 60 percent by weight of zinc, 0.1to 1 percent by weight of nickel, and the remainder being tin may beused.

In this way, inclusion of 30 to 60 percent by weight of zinc makestin-zinc-nickel type solder or tin-zinc-copper type solder with a solidphase temperature of 199 degrees C. and a liquid phase temperature of360 degrees C. or greater, which are preferable for a current fuse.

Incidentally, tin-zinc-nickel type solder and tin-zinc-copper typesolder have fluidity when reheated in the reflow process for mountingthe chip-type fuse on the printed circuit board. Therefore, expansion ofair inside the chip-type fuse may cause malfunctions such as washing outthe solder. Accordingly, fluidity of solder must be reduced.

Since specific gravity of aluminum is low, aluminum solidifies on thesurface of a joint when it melts. In addition, since aluminum is easilyoxidized, a solid oxide film is formed on the surface of the joint.Therefore, insulation of the fuse increases when becoming anopen-circuit, resulting in an arc-suppressing operation.

In addition, since aluminum is effective in decreasing the fluidity ofsolder, 0.01 to 0.5 percent by weight of aluminum is added to solder inthe present embodiment.

However, if the added amount of aluminum is less than 0.01 percent byweight, the influence on fluidity is small, resulting in no prescribedeffectiveness. On the other hand, if the added amount of aluminum isgreater than 0.5 percent by weight, conductivity decreases due tooxidation of aluminum when the solder is left in a high-temperatureenvironment. From the above, with the embodiment, a very small addedamount of aluminum, which is effective for fluidity, is used, and theamount of additive is determined based on the solder use conditions.

In this way, inclusion of 30 to 60 percent by weight of zinc leads to apreferable solid phase temperature for the current fuse. Alternatively,it is preferable that tin-zinc solder containing 40 percent by weight ofzinc with characteristics such as heat resistance equivalent to those ofthe conventional tin-zinc solder is used. Therefore, in the current fuseof the embodiment, tin-zinc solder containing 40 percent by weight ofzinc is used.

The evaluation results of various percentages of copper, nickel, andaluminum contents to be contained in that tin-zinc solder are shown inFIG. 6. ‘Processability’ in FIG. 6 means workability of inserting asolder joint into an electrode cap. Moderate flexibility is favorablebecause the solder joint fits to the interior of the electrode cap. Inaddition, ‘solder manufacturability’ means workability of manufacturinga joint by melting a solder material.

In addition, ‘heat resistance’ means change in resistance due to changein temperature; and ‘wettability’ means adherability of solder to abonding target. Poor wettability causes solder to peel off of thesurface of the bonding target.

FIG. 5 is a graph showing the results of comparing the changes in theresistance value of a fuse in which a conventional solder is used andchanges in the resistance value of a fuse in which a solder according tothe embodiment is used. A performance test is carried out for the solderaccording to the embodiment, which contains 40 percent by weight ofzinc, 1 percent by weight of copper, 0.5 percent by weight of nickel,and the balance consisting of tin, and conventional solder, whichcontains 10 percent by weight of tin and 90 percent by weight of lead.

Note that the rate of change in the resistance value is examined underthe conditions of immersion in molten solder at a temperature of 260degrees C. for ten seconds repeated five times. As is seen from FIG. 5,in the case of the fuse using solder according to the embodiment, therate of change in the resistance value is 2% or less at most. This is acharacteristic almost equivalent to that of the fuse using theconventional solder containing lead (indicated by a dashed line in thedrawing).

Such characteristic can be obtained based on the following reasons. Inshort, the solder according to the embodiment has low fluidity in asolid-liquid coexistence state. Therefore, even if the solder is heatedwhen the fuse is mounted on the printed circuit board, the shape ofsolder (joint) connecting the fuse and the electrode does not change,and the connection is maintained.

This is because the solder according to the embodiment being soldered inan atmosphere of oxygen steadily oxidizes the surface thereof due toheat generated during soldering, and forms a solid oxide film, resultingin decrease in fluidity thereof.

In addition, since the solder joint according to the embodiment cannever be exposed to oxygen due to a special flux, progression ofoxidation of the solder joint after being soldered stops. This is onereason why the above-mentioned characteristic can be obtained.

A method of manufacturing a current fuse with the above-mentionedconfiguration is described forthwith while referencing FIG. 7. FIG. 7 isa flowchart describing the steps of manufacturing the current fuseaccording to the embodiment.

To begin with, in step S1, the fuse wire 1 is aligned with the groove 8at the bottoms of the concave portions 11 of the casing main body 2 andstretched across the opening 9. At this time, a certain tension may beapplied to the fuse wire 1 in order to temporarily adhere to the groove8 using an adhesive resin. This omits application of a certain tensionafter the temporary adhesion. Note that there may not be need totemporarily adhere or apply tension when providing the fuse wire in thegroove 8 according to the shape of the fuse wire.

Next, in step S2, the lid 3 is attached to the opening 9, and theexposed ends of the fuse wire are bent. This makes the exterior of thecurrent fuse shown in FIG. 2, in other words, the bent portions 10 ofthe fuse wire 1 protrude from the ends of the substrate. Note that inthis case, the lid 3 and the casing main body 2 are bonded together withan adhesive, and the adhesive is hardened by heating at a giventemperature for a given period of time (e.g., at a temperature of 150degrees C. for approximately 15 minutes).

Next, in step S3, the solder chips 5 with the above-mentionedcomposition are aligned and inserted into the interior 6 of therespective metallic caps (electrodes) 4 so as to cover the bent portions10 of the fuse wire 1, respectively, when the electrodes 4 are fit overthe substrate.

In addition, in step S4, the electrodes 4 into which the solder chips 5are inserted, respectively, are then pressed into and attached to bothends of the substrate. At this time, it is desirable that the solderchips 5 are provided at the bent portions 10. The exterior of the fuseover which the electrodes 4 are fit as described above is shown in FIG.3.

Next, in step S5, the center portion (indicated by a dashed line 7 inFIG. 1) of each electrode 4 is heated by a heater at a temperaturebetween a solder solid phase temperature and a solder liquid phasetemperature, for example, at a temperature of 350 degrees C. forapproximately 3 seconds. This melts the solder chips 5 which have beeninserted into the electrodes 4 in advance.

As a result, the solder chips 5 are soldered (fusion bonded) to theabove-described bent portions 10 of the fuse wire 1, and the fuse wire 1and the electrodes 4 are soldered in an atmosphere of oxygen, resultingin establishment of an electrical connection therebetween (in step S6).

Note that the lid 3 is attached to the casing main body 2 by bondingtogether with an adhesive such as an epoxy, and heating at a giventemperature for a given period of time (e.g., at a temperature of 150degrees C. for approximately 15 minutes). The electrodes 4 are pressedin and attached after the adhesive has hardened.

As described above, according to the embodiment, connection betweenelectrodes and a fuse wire can be secured by inserting a solder chipinto the interior of each electrode in advance before pressing theelectrodes into the ends of the substrate of the fuse, and then heatingthe exterior of the electrodes to melt the solder, and keeping a goodelectrical joint condition at the junctions thereof.

In addition, usage of tin-zinc-nickel type solder that does not containharmful materials such as lead as the solder chip to be inserted intothe interior of each electrode for connection with the fuse line, notonly prevents adverse influence on the environment and human health andalso adverse influence on other electronic components to be mountedtogether with the current fuse, but also allows to maintain the internalcondition of the current fuse even if overheated when the fuse ismounted on the printed circuit board.

Furthermore, addition of aluminum as a solder alloy material improvesheat resistance and fluidity of solder, resulting in improvement ofsolderability, and prevention of change in the shape of the solder jointdue to reheating.

In addition, the fuse according to the embodiment allows a sufficientjoint between the electrode and the fuse wire by heating the electrodeat a temperature of 350 degrees C. As a result, soldering is completedat a temperature less than the liquid phase temperature even withconsideration for heat radiation from the ceramic substrate.Accordingly, soldering is possible even in a solid-liquid coexistencestate at a temperature between the solid phase temperature and theliquid phase temperature, thereby preventing degradation of plating onthe surface of the electrode.

Unavoidable impurities specified by Japanese Industrial Standards (JIS)may be included in the composition of the lead-free solder describedabove. However, even the case of such unavoidable impurities beingcontained in the lead-free solder does not depart from the scope of theappended claims of the present invention.

As described above, the present invention provides a current fuse, whichdoes not have an adverse influence on the environment due to harmfulmaterials, and has excellent characteristics such as solderability on aprinted circuit board, and a method of making the same.

While the invention has been described with reference to particularembodiments, further modifications and improvements which will occur tothose skilled in the art, may be made within the purview of the appendedclaims, without departing from the scope of the invention in its broaderaspect.

1. A method of making a current fuse by soldering a fuse wire and electrodes using lead-free solder at a temperature between a solid phase temperature and a liquid phase temperature of said lead-free solder.
 2. The method of making a current fuse according to claim 1, wherein said lead-free solder contains at least 30 to 60 percent by weight of zinc, 0.1 to 2 percent by weight of copper, 0.1 to 1 percent by weight of nickel, with the remainder percent by weight being tin.
 3. The method of making a current fuse according to claim 2, wherein said lead-free solder further contains 0.01 to 0.5 percent by weight of aluminum. 